The present invention relates to an electrophotographic copier, facsimile machine, laser printer or similar image forming apparatus which reproduces an image on a paper sheet by transforming an image signal representative of the image into an image forming signal, scanning a photoconductive element with a spot beam in the form of a laser beam being modulated by the image forming signal to form an electrostatic latent image on the photoconductive element, developing the latent image to render it visible, and transferring the visible image to a paper sheet.
An image forming apparatus whose reproducibility is not satisfactory with respect to halftone may reproduce false halftone by using a dither method, density pattern method or similar implementation heretofore proposed (e.g. Japanese Patent Laid-Open Publication (Kokai) No. 57-76977). A predominant procedure for the false reproduction of a halftone image currently practiced is comparing a halftone image signal or light/shade image signal with a threshold signal or dither signal to digitize it with respect to two levels or a plurality of limited levels, and changing the dot size or the dot density. Assuming that the threshold signal is represented by a 4.times.4 matrix, an image forming apparatus capable of rendering only two levels cannot reproduce more than seventeen tones. This results in a noticeable difference in density between successive tones and therefore in a false contour. Although this problem may be overcome by increasing the matrix, this kind of scheme brings about another problem that the resolution of an image becomes poor.
In an image forming apparatus of the type using a laser beam, to guarantee a satisfactory resolution, one dot of a laser beam or spot beam for illuminating a photoconductive element is subjected to pulse width modulation, i.e., the beam turn-on time per dot is changed to render several different tones by means of one pixel. However, when the power of the laser beam is changed by the deterioration of a laser and the contamination of mirrors, lenses and other components of optics or when the sensitivity of a photoconductive element is changed due to the fatigue of the element itself and the variation of ambient temperature, the area of a dot is changed. This effects the density of an image to be reproduced and degrades the tone characteristic, resulting in poor reproducibility.
A prior art approach available for eliminating the above drawback is to detect the number of times that an image forming apparatus has repeated its operation and the fatigue of a photoconductive element so as to control a bias voltage for development based on the detected values. Although this kind of scheme can uniformly change the overall density distribution (low densities to high densities), it is incapable of changing the tone characteristic. Another approach heretofore proposed in relation to an apparatus of the type using a laser beam is to change the density of an image by changing a laser beam output. While this scheme is of course not capable of offering high image quality constantly, it is advantageous over the previously stated bias voltage scheme in that as the image density is increased due to aging, the laser beam output and therefore the image density is decreased to suppress the fatigue of the photoconductive element.